1. Field of the Invention
This invention relates to novel silacyclohexane compounds and also to liquid crystal compositions comprising the compounds and devices comprising the compositions.
2. Description of the Prior Art
The liquid crystal display devices make use of optical anisotropy and dielectric anisotropy of liquid crystal substances. Depending on the mode of display, a variety of display systems are known including those of a twisted nematic type (TN type), a supertwisted nematic type (STN type), a super birefringence type (SBE type), a dynamic scattering type (DS type), a guest/host type, a type of deformation of aligned phase (DAP type), and an optical mode interference type (OMI type). The most popular display device is one which is based on the Schadt-Helfrich effect and has a twisted nematic structure.
Although the properties of the liquid crystal substances used in these liquid crystal devices depend, more or less, on the type of display, it is commonly required that the liquid crystal substances have a wide range of temperatures at which they are able to work as a liquid crystal and that they be stable against moisture, air, light, heat, electric field and the like. Moreover, the liquid crystal substances should desirably be low in viscosity and should ensure a short address time, a low threshold voltage and a high contrast in cells.
Liquid substances which can satisfy all these requirements have never been known when used singly. In practice, several to ten and several liquid compounds or latent liquid crystal compounds are mixed and used in the form of a mixture. To this end, it is important that constituent components be readily compatible with one another.
Typical of such constituent components are ester compounds having a relatively high nematic-isotropic transition temperature, T.sub.NI. The ester compounds include those compounds having phenyl ester structures of cyclohexylbenzoic acid of the following formulas. ##STR3## wherein R represents an alkyl group having from 1 to 8 carbon atoms as set forth in Japanese Patent Publication No. 60-17777. ##STR4## wherein R and R' independently represent an alkyl group having from 1 to 8 carbon atoms as set forth in Japanese Patent Publication No. 60-17777. ##STR5## wherein R and R' independently represent an alkyl group having from 1 to 8 carbon atoms as set forth in Japanese Patent Publication No. 60-17777. ##STR6## wherein R represents an alkyl group having from 1 to 9 carbon atoms as set forth in Japanese Patent Publication No. 61-24382. ##STR7## wherein R represents an alkyl group having from 1 to 10 carbon atoms as set out, for example, in Japanese Patent Publication No. 1-42261. ##STR8## wherein R represents an alkyl group having from 1 to 10 carbon atoms and X and Y independently represent F or Cl as set out in Japanese Patent Publication No. 2-51893. ##STR9## wherein R is an alkyl group having from 1 to 9 carbon atoms as set out in Japanese Patent Publication No. 2-56343. ##STR10## wherein R is an alkyl group having from 1 to 10 carbon atoms.sub.-- as set out in Japanese Patent Publication No. 2-47455. ##STR11## wherein R is an alkyl group having from 1 to 10 carbon atoms.sub.-- and X is F or Cl as set out in Japanese Patent Publication No. 63-53178. ##STR12## wherein R.sup.1 represents an alkyl group having from 1 to 8 carbon atoms, and R.sup.2 represents R, RO or CN in which R is an alkyl group having from 1 to 8 carbon atoms as set out in Japanese Patent Publication No. 60-17777.
As the liquid crystal display devices recently have wider utility, the characteristic properties required for the liquid crystal materials become severer along with a diversity of drive systems and working modes being in progress. In particular, liquid crystal materials should have a high response speed and, for on-vehicle needs, should have a nematic phase extended to a high temperature region from the standpoint of use conditions. For the extension of the nematic phase to a high temperature region, it is sufficient to add a liquid crystal compound having a high nematic-isotropic transition temperature, T.sub.NI, as a constituent component. Known components having a high T.sub.NI value include, for example, 4,4"-substituted terphenyl, 4,4'-substituted biphenylcyclohexane and 4,4'-substituted cyclohexylbiphenylcyclohexane. However, these compounds because the viscosity of mixed liquid crystal to increase, thus bringing about a response speed being lowered.